As the insoluble anodes used in an electrolytic process of electroplating or the like, lead or a lead alloy has been conventionally used in most cases. However, there are problems such as environmental pollution by lead eluted from this lead based anode. Therefore, there have been developed clean insoluble anodes in place of the lead based anode, one example of which is an anode using valve metals, above all, a titanium based anode using titanium.
In the titanium based anode, the surface of an anode substrate comprising pure titanium or a titanium alloy (these are generically referred to as titanium) is covered in layers by an electrocatalyst comprising iridium oxide or the like. Since a pyrolysis method is used for covering this insoluble anode with the electrocatalyst, crack takes place in the electrocatalyst layer thus formed, so that the electrocatalyst layer tends to be peeled off. Further, even if not going far enough to peeling-off, the electrocatalyst layer floats from the surface of the anode substrate, and an anode function tends to be lost. Therefore, there is a fundamental problem of short anode life.
To solve the problem of lifetime in the titanium based anode, generally, the surface of an anode substrate is roughened beforehand by a blast treatment or an etching treatment, and through an anchor effect thereby generated, an electrocatalyst layer has been fixed rigidly to the surface of the substrate. Further, alternatively, it is proposed that a medium layer comprising a valve metal other than titanium, such as tantalum, is interposed between an anode substrate and an electrocatalyst layer (see Patent documents 1 and 2).
Patent document 1: JP-A-7-229000 (1995)
Patent document 2: JP-A-8-109490 (1996)
The lifetime of an anode is lengthened by these measures. However, due to an electrolytic process accompanying a cathodizing phenomenon of an anode, consumption of an anode at the parts where cathodizing occurs rapidly proceeds, and since the lifetime of the whole anode is determined by this partial consumption, the present situation is that the expected effect cannot be sufficiently obtained. The cathodizing phenomenon of an anode will be briefly explained below.
For example, in an electroplating line of a steel plate, in order to plate both surfaces of a steel plate, two pieces of anodes are placed opposite to each other, and a steel strip being a cathode passes between the anodes, thereby plating metal is deposited on both surfaces of the steel strip. Herein, regarding the width of the two pieces of anodes placed opposite to each other (size in a direction perpendicular to the traveling direction of steel strip), since there are various widths of steel strips passing therebetween, it is set to a maximum width of the steel strips. Hence, when a steel strip with a smaller width than the maximum width passes, electrodes will directly face each other at the side edges in both sides of the anodes. Further, when metal plating with different thicknesses in both surfaces of a steel plate is conducted, a potential difference occurs between the two pieces of anodes, and in an anode of a lower potential side, the side edges where electrodes face directly each other act as a cathode.
This is the cathodizing phenomenon of an anode, and in the side edges of the anode suffering from this phenomenon, consumption of the electrocatalyst proceeds rapidly compared to the center part facing a steel strip, and this rapid consumption of the electrocatalyst in the side edge dominates a lifetime of the whole anode.
In view of such situations, it is an important technical object regarding an insoluble anode to suppress local consumption of the electrocatalyst involved with the cathodizing phenomenon of an anode, and as a means for achieving the technical object, a layer thickness of the electrocatalyst is made thicker in a part causing the cathodizing phenomenon than in other parts (see Patent document 3).
Patent document 3: JP-A-10-287998 (1998)
To suppress local consumption of the electrocatalyst involved with the cathodizing phenomenon of an anode, it is effective to increase a layer thickness of an electrocatalyst layer. However, it cannot be said that a consumption-suppressing effect is sufficient for the degree of the increase. The reason is that, in spite of a good amount of electrocatalyst left on an anode substrate, the electrocatalyst floats from the surface of the substrate, or a passive layer is formed between both of them, often causing an anode function to be lost. Moreover, when the layer thickness of an electrocatalyst layer is increased, there are also problems that the peeling off and drop-off of the electrocatalyst become remarkable.
In addition thereto, an increase in layer thickness of an electrocatalyst layer accompanies a large increase in costs. Namely, the electrocatalyst layer is formed up to a predetermined layer thickness by repeating a so-called bake coating where an electrode covering liquid is applied and calcined. To increase the layer thickness, it is necessary to increase the repeating number of the bake coating, which leads not only to an increase in the amount of expensive electrocatalysts used but also to a marked increase in the number of processes.
Further, when a lifetime of an anode is intended to extend, there have been many instances that electrocatalysts are improved, but the effect has been small for the lot of costs.
As described above, it has been desired to develop an economical long-life insoluble anode capable of maintaining an anode function stably for a long time when it is used at a part where the cathodizing phenomenon of an anode is caused, and also capable of reducing the amount of electrocatalysts used as much as possible.